1. Field of the Invention
The invention relates to a control system for a fuel direct injection type of automobile engine.
2. Description of the Related Art
Fuel direct injection engines are changeable in fuel injection mode according to engine operating conditions. Such a fuel direct injection engine described in, for example, Japanese Unexamined Patent Publication No. 8-189405 is changeable among different injection modes, namely a compression stroke injection mode or latter injection mode taking place during engine operation in a range of lower engine loads where the engine has a demand for a relatively small amount of fuel, an intake stroke injection mode or early injection mode taking place during operation in a range of higher engine loads where the engine has a demand for a relatively large amount of fuel, and a split injection mode taking place during operation in a range of moderate engine loads which is a transitional range from the lower load range to the higher load range and vice versa. The term "compression stroke injection" or "latter injection" mode refers to a mode in which fuel injection is effected at a time in the latter half of an compression stroke of the engine, the term "intake stroke injection" or "early injection" mode refers to a mode in which fuel injection is effected at a time in a period including a full intake stroke and the early half of a compression stroke of the engine, and the term "split injection" mode refers to a mode in which fuel injection is separately effected in an intake stroke and in a compression stroke. This fuel direct injection engine is controlled in operation with a control value such as an ignition time which is determined from a control map according to engine operating conditions and varied according to the injection modes.
Typically, such an ignition control map comprises a great number of intersection points of a grid pattern representing the relationship between parameters concerning engine operating condition and control value. If the parameter is in a position between intersection points, the control value is calculated by interpolation with the aid of values at adjacent intersection points between which the parameter is. For example, as seen in FIG. 9 showing a prior art control map regarding the relationship between engine load and ignition time which defines a latter injection mode area and a split injection mode area, if the present engine load ac is in close proximity to the boundary between the two injection mode areas and takes an in-between position limited by an extreme intersection point Ta in the latter injection mode area and an extreme intersection point Tb in the split injection mode area, an ignition time T for the engine load ac is calculated by interpolation with the aid of values at the intersection points Ta and Tb. That is, the ignition time T takes an intermediate value between the values at the intersection points Ta and Tb. The ignition time varies linearly or continually between the values at the intersections Ta and Tb with a change in engine load.
A transition, for example, from the latter injection mode area to the split injection mode area is generally accompanied by a significant change in engine operating condition which must be too sudden to timely achieve the calculation of ignition time by interpolation, leading to a delay in timely ignition, and hence to unstable engine operation. At a transition from the latter injection mode to the split injection mode, it is necessary to advance ignition timing to cause early combustion depending on expansion of an area for diffusion of injected fuel partly allocated to homogeneous charge combustion. In other words, a minimum advance for best torque (MBT) is needed to be shifted to an advanced side. If the shift of ignition timing toward the advanced side is slow, ignition is hard to occur in well response to a rapid switch of injection mode, which always causes torque shocks.